Magnetostrictive devices, which generate strain through application of a magnetic field, are employed as magnetostrictive sensors, magnetostrictive vibrators, and similar devices for generating or detecting displacement with precision. Conventionally, RE-containing intermetallic compounds such as TbFe2, DyFe2, and SmFe2 are employed as magnetostrictive materials. However, these intermetallic compounds generate only minute displacement, and precise control of the generated displacement is difficult. Therefore, these compounds cannot be applied to magneto-displacement devices for controlling minute displacement.
Meanwhile, GGG (gallium gadolinium garnet) is known to be a magnetic refrigerant which may be applicable to magnetic refrigerators or similar devices. However, GGG has not yet been used in a commercial product, because of its poor refrigeration efficiency upon application of a weak magnetic field provided by a permanent magnet.
In recent years, an RE-containing alloy having an NaZn13 phase (hereinafter referred to as “NaZn13-type RE-containing alloy”) has been found to exhibit large magnetostrain and high magnetocaloric effect. By virtue of these properties, this type of alloy is thought to be a candidate magnetostrictive material, magnetic refrigerant, etc.
Specifically, La(FeaSi1-a)13 (0.84≦a≦0.88) is disclosed to exhibit a magnetostrain as large as about 0.4% at 200 K under ≧4T (see, for example, non-patent reference 1).
It is also disclosed that, when the above compound is transformed into La(FeaSi1-a)13Hb (0.84≦a≦0.88, 1.0≦b≦1.6) through hydrogen absorption or similar treatment, Curie temperature can be controlled and magnetocaloric effect can be maintained at a high level (see, for example, non-patent reference 2).
Conventionally, NaZn13-type RE-containing alloys such as La(FeaSi1-a)13 (0.84≦a≦0.88) have been produced by weighing alloy raw materials such as high-purity La, Fe, Si, etc., so as to attain a desired alloy composition and mixing; melting the mixture through arc melting; and heating the product for a considerably long period of time (e.g., at 1,050° C. for 1,000 hours) in order to remove an undesired phase (see, for example, non-patent reference 2).
In the conventional method for producing NaZn13-type RE-containing alloys, the long-term heat treatment step for removing an undesired phase lowers productivity and increases costs in the production of NaZn13-type RE-containing alloys, devices employing the alloys, and other products employing the alloys.
Among RE-containing alloys having an NaZn13 structure, an La—Fe—Si alloy has been found to exhibit magnetic phase transition concomitant with a large entropy change in accordance with a change in the external magnetic field and to have no temperature hysteresis in the magnetocaloric effect. By virtue of these properties, this type of alloy is considered a candidate magnetic refrigerant.
The magnetic phase transition temperature of the La—Fe—Si alloy can be controlled by absorbing hydrogen into the alloy, and the change in entropy does not decrease even when hydrogen is absorbed (see non-patent reference 2). Therefore, when the magnetic phase transition temperature of the alloy is controlled to approximately room temperature and a permanent magnet is employed to generate a magnetic field, the alloy can be used as a magnetic refrigerant which can work at about room temperature.
In addition, this type of alloy, which exhibits a large, isotropic volume change under application of an external magnetic field, is also considered a candidate magnetostrictive material (see non-patent reference 1).
A conventionally known method for producing an La—Fe—Si alloy having an NaZn13 structure includes arc-melting raw material metals (i.e., La, Fe, and Si), to thereby form an alloy ingot; heating the alloy ingot in an inert atmosphere at 1,000 to 1,200° C. for 240 hours to 1,000 hours, to thereby form a mother alloy; re-melting the mother alloy; atomizing the formed molten alloy in an atmosphere for cooling, to thereby produce spherical particles; and absorbing hydrogen into the particles, to thereby control the magnetic phase transition temperature to a predetermined level (see patent reference 1).
However, the aforementioned conventional method for producing an RE-containing alloy powder has a drawback. Namely, since the method includes a long-term heat treatment and two melting steps, production costs increase and oxygen content of the alloy increases, even though low-cost material is used.
[Non-Patent Reference 1]
Maya FUJITA and Kazuaki FUKAMICHI, Itinerant-Electron Metamagnetic La(FexSi1-x)13 Compounds, “Solid-State Physics,” Vol. 37, No. 6, (2002), p. 419-427
[Non-Patent Reference 2]
Maya FUJITA, Shun FUJIEDA, and Kazuaki FUKAMICHI, Large Magnetic Volume and Magnetocaloric Effect of Itinerant-Electron Metamagnetic La(FexSi1-x)13 Compounds, “Materia,” Vol. 41, No. 4, (2002), p. 269-275
[Patent Reference 1]
Specification of Japanese Patent Application Laid-Open (kokai) No. 2003-96547